During the next year we propose to continue our work on a) the molecular mechanisms of polypeptide chain termination and b) the isolation and characterization of animal cells that carry suppressor mutations. Our previous work on the former problem suggests that release factor (RF; the cytoplasmic protein required for termination of polypeptide synthesis) contains two non-identical subunits of 54,000 and 57,000 molecular weight. The contribution of these subunits to RF activities and the association of RF with the ribosome will now be studied using antibodies directed against the individual subunit proteins. As part of the later problem we have isolated mutant and revertant Chinese hamster cells that produce detectably altered hypoxanthine-guanine phosphoribosyl transferase. These altered proteins will now be purified using recently developed immunological techniques and the changes in primary structure studied so that we can gain additional information about the structure and function of the enzyme and the genetic changes responsible for the variant phenotypes. Bibliographic references: Inhibition of Peptide Chain Termination by Antibodies Specific for Ribosomal Proteins, J. Mol. Biol., 1975, 93, 375-389. Tate, Warren P., and Caskey, C. Thomas, The Mechanism of Peptide Chain Termination, Molecular Cell Biochemistry, 5:115-126;